Because blood is prone to viral contamination, and because donated blood has a limited shelf life, donated blood appears to be in constant short supply. In response, much effort has been focused on the development of compositions commonly referred to as "blood substitutes" or "artificial blood". Such compositions transport oxygen to tissues, as do red blood cells, but such compositions generally lack the ability to perform the metabolic, regulatory, and protective functions of blood. Thus, these compositions are more appropriately termed "gas carriers".
Various gas carriers are known to those skilled in the art. One class of gas carriers includes hemoglobin, modified hemoglobin (polymerized, conjugated, crosslinked, or phospholipid-encapsulated), recombinant hemoglobin, and hemoglobin derivatives. Another class of gas carriers comprises liquid perfluorochemicals. Typically, perfluorochemicals (PFCs; also known as "perfluorocarbons") are liquids that dissolve oxygen and that are composed of 8 to 10 carbon atoms per molecule. Oxygen, carbon dioxide, and nitrogen are examples of gases that are highly soluble in PFCs. For intravascular use, PFCs are generally administered in emulsions, with emulsifiers such as egg yolk phospholipid or poloxamers, because PFCs are insoluble in water or an aqueous environment. Thus, as small droplets of about 0.1 to 0.2 microns in diameter, PFCs are not metabolized but are excreted by the lungs. However, depending on the factors such as particle size, viscosity, surface tension, and chemical composition of the PFC emulsion, stability in the presence of blood and other biological fluids and efficiency/efficacy of gas transport to tissues varies. As a result, various liquid PFC emulsions are being investigated for characteristics such as elimination, distribution, tissue retention, and physiological changes after administration. Such factors and characteristics affect the ability of PFCs to be used in various medical applications. Studies to date indicate that significant amounts of PFC emulsion need be injected to sufficiently supplement or replace the oxygen carrying capacity of hemoglobin. Further, in the body, many liquid PFC emulsions become sequestered in organs such as the spleen and liver.
Emulsions of liquid PFCs have been described to be potentially useful as oxygen carriers for various medical applications including as a "blood substitute" in the treatment of heart attack, stroke, and organ perfusion; as adjuvants to coronary angioplasty; and in cancer radiation treatment and chemotherapy. PFCs said to be useful in such applications are described, for example, in U.S. Pat. Nos. 5,403,575; 4,868,318; 4,866,096; 4,865,836; 4,686,024; 4,534,978; 4,443,480; 4,423,077; 4,252,827; 4,187,252; 4,186,253; 4,110,474; and 3,962,439. Such liquid PFC emulsions include perfluorooctyl bromide, perfluorooctyl dibromide, bromofluorocarbons, perfluoroethers, Fluosol DA.TM., F-44E, 1,2-bisperfluorobutyl-ethylene, F-4-methyl octahydroquinolidizine, 9 to 12 carbon perfluoro amines, perfluorodecalin, perfluoroindane, perfluorotrimethyl bicyclo3,3,1! onane, perfluoromethyl adamante, perfluorodimethyl adamantane.
Recently, microbubbles have been developed for use as contrast-enhancing agents for ultrasonic imaging of the heart and blood vessels. One commercially available preparation of such microbubbles is produced by sonication of albumin solution (see for example U.S. Pat. Nos. 4,718,433; 4,774,958 and 4,957,656). These microspheres are made by sonicating protein solutions, such as 5% human albumin. A method of preparing stable suspensions of microbubbles using various protein solutions, and their use in echographic investigation is also disclosed in U.S. Pat. No. 5,310,540. Another species, lipid-coated microbubbles, their method of preparation, and their use in ultrasound and magnetic resonance imaging techniques, is described in U.S. Pat. No. 5,215,680. A method of making an ultrasound contrast agent, comprising gas-filled microbubbles wherein the gas is a halogenated hydrocarbon, with improved resistance against collapse due to pressure is disclosed in U.S. Pat. No. 5,413,774.
Other microbubbles are formed from PFCs (U.S. Pat. No. 5,409,688) in methods for ultrasound imaging (U.S. Pat. No. 5,393,524). PFCs that are disclosed as being useful for creating microbubbles include dodecafluoro-pentane (DDFP), sulfur hexafluoride, pentane, hexafluoropropylene, octafluoropropane, hexafluoroethane, octafluoro-2-butyne, hexafluorobuta-1,3-diene, isoprene, octafluorocyclobutane, decafluorobutane, cis-2-pentene, dimethyl sulfide, ethylarsine, bromochlorofluoromethane, trans-2-pentene, 2-chloropropane, hexafluorodisulfide, ethylmercaptan, diethylether, ethylvinylether, valylene, trisfluoroarsine, furfuyl bromide, cis-propenyl chloride, bytyl fluoride, 1,1 dichloroethane, isopropyl methyl ether, isopropylamine, methylfomate, 2-acetyl-furan, ethylenefluoride, 1-pentene, isopropylacetylene, perfluoropentane, isopentane, vinyl ether, 2-butyne, 1,4-pentadiene, tetramethyl silane, dimethyl phosphine, dibromodifluoromethane, 2-chloro-propene, difluroiodomethane, acetaldehyde, trimethyl boric, 3-methyl-2-butene, 1,1 dimethylcyclopropane, aminoethane, vinyl bromide, disilanomethane, trichlorofluoromethane, bromofluoromethane, trifluorodichloroethane, perfluoropentene, and other fluorine containing hydrocarbons (U.S. Pat. No. 5,409,688).
Features which make the various species of microbubbles (e.g., protein-coated, lipid-coated or surfactant-coated microbubbles, and microbubbles stabilized by gas that permeates very slowly across the bubble/liquid interface) useful as contrast media for ultrasound imaging are that such microbubbles can be prepared to a particular size range (e.g., 1 .mu.m to 6 .mu.m); and are stable in physiological solutions for at least several minutes and up to several hours.
However, whether such microbubbles could be used as gas carriers, such as to transport oxygen (O.sub.2) to tissues, was not known or described before the present invention. For example, for microbubbles to be used to transport O.sub.2, several practical and functional requirements must be demonstrated: (1) the microbubbles must be of a sufficiently small size to pass through capillaries; (2) ordinary bubbles of such size dissolve rapidly under the pressures of surface tension, and thus, the microbubbles must be sufficiently stabilized to persist in the bloodstream; (3) the microbubbles must be permeable to oxygen and other respiratory gases; and (4) desirably, the microbubbles should unload O.sub.2 in tissues at high enough PO.sub.2 to be physiologically or medically effective. A potential disadvantage of using such microbubbles as gas carriers is the danger of embolization if infused microbubbles are, or become, too large in size.